1. Field of the Invention
The present invention generally relates to an image processing apparatus and an image processing method for processing image data for a unit area of a recording medium, so as to form an image on the unit area, with a plurality of relative scans of a recording head with respect to the unit area of the recording medium, or a relative scan of a plurality of recording heads with respect to the unit area of the recording medium.
2. Description of the Related Art
As an example of recording methods employing a recording head having a plurality of recording elements, an inkjet recording method for forming dots on a recording medium by ejecting ink from each of the recording elements is known. Such inkjet recording apparatuses can be categorized into either a full-line type and a serial type based on a difference in the configurations thereof.
Full-line type recording apparatuses employ recording heads in which a plurality of recording elements corresponding to the width of a recording media is arranged. The recording apparatuses form an image by conveying a recording medium in a direction orthogonal to an arrangement direction of the recording elements, while ejecting ink from the recording heads. Such full-line type recording apparatuses may be suitable for office use because they can output images at a relatively high speed.
On the other hand, serial type recording apparatuses incrementally form an image by repeating a main recording scan for moving a recording head that ejects ink, and a conveying action for conveying a recording medium in a direction orthogonal to this main recording scan direction. Such serial type recording apparatuses may be suitable for personal use because they can be manufactured into a relatively small shape at low cost.
Regardless of whether the recording apparatuses are the full-line type or the serial type, an amount of ejected ink and an ejecting direction can vary between recording elements in a recording head including the plurality of recording elements arranged therein. Such variability may cause color density unevenness and streaks in images.
A multipass printing method is known as a technique for reducing such image degradation. In the multipass printing, image data to be recorded on a unit area of a recording medium is typically divided into pieces of image data corresponding to a plurality of scans, and the divided pieces of image data are sequentially recorded with the plurality of scans, whereby the image to be recorded on the unit area may be completed. According to such a multipass printing method, it may be possible to reduce the image degradation resulting from variability in the ejection of each recording element. As a result, relatively even and smooth images can be obtained. The advantages of such multipass printing may be enhanced as the number of passes, namely, the number of recording elements employed in recording of one scan raster, increases. However, the printing speed may also reduce as the number of passes increases. Accordingly, serial type recording apparatuses often prepare a plurality of recording modes that employ different numbers of passes.
Meanwhile, the above-described multipass printing method can also be applied to full-line type recording apparatuses. More specifically, when a plurality of recording element lines are provided for ink of an identical color along a direction in which a recording medium is conveyed (hereinafter, referred to as a conveyance direction) as shown for example in FIG. 1, an image can be recorded by allotting a raster extending in the conveyance direction to the plurality of recording elements. As a result, even if variability in ejection is caused between the recording elements, an effect thereof can be reduced.
At the time of such multipass printing, image data may be distributed to each recording scan of the serial type recording apparatuses or to each recording head of full-line type recording apparatuses. In the related art, such distribution is performed using mask patterns in which print-permitting pixels (1), regarding which printing of dots is permitted, and non-print-permitting pixels (0), regarding which printing of dots is not permitted, are arranged.
FIG. 18 is a schematic diagram showing examples of the mask patterns employable in two-pass multipass printing. Here, black areas represent print-permitting pixels (1), whereas white areas represent non-print-permitting pixels (0). Patterns 1801 and 1802 correspond to mask patterns that may be employed in the first and second recording scans, respectively. The patterns 1801 and 1802 may have a mutual complementary relationship.
By performing logical multiplication of such a mask pattern and binary image data, the binary image data may be divided into two pieces of binary image data to be recorded in respective recording scans. For example, as shown in FIG. 2, by dividing image data representing dots to be recorded on a unit area using the mask patterns (1801 and 1802) shown in FIG. 18, divided image data may be generated for both the first pass and the second pass. Since the divided pieces of binary image data corresponding to different scans also have a complementary relationship when a data division method (mask-employed division method) is performed using the mask patterns having the mutual complementary relationship, a probability that dots recorded in different scans overlap one another may be relatively low. Accordingly, a relatively high color density resulting from a relatively high dot coverage ratio can be realized, and improved graininess can be provided.
Now, there is increasing demand for higher image quality while such multipass printing is employed. In such a circumstance, there is a need for reduction in a color density change and a color density unevenness that may result from a shift of the recording position (registration) of each printing scan or of each recording element line. The shift of the recording position of each printing scan or of each recording element line may be caused by an alteration in a distance (paper distance) between a recording medium and an ejection orifice surface, and an alteration in a conveyed distance of the recording medium.
For example, referring to FIG. 2, a case is considered where positions of a plane of dots (◯) recorded in a preceding recording scan and a plane of dots  recorded in a following recording scan are shifted by an amount equivalent to one pixel in a main scanning direction or a sub scanning direction. In this case, the dots (◯) recorded in the preceding recording scan and the dots  recorded in the following recording scan completely overlap one another, and a blank area is exposed, due to which the color density of the image is decreased. If a distance between neighboring dots or an overlapping amount changes, even if the shift amount is not as large as one pixel, a coverage ratio of the dots with respect to the blank area may nonetheless change. This change in the coverage ratio can cause a change in color density of the image. The change in the color density of the image may then be recognized as color density unevenness.
Accordingly, as demand for higher image quality continues to increase, there is a need for an image data processing method that is employed in multipass printing and is capable of coping with a shift of the recording positions of the planes caused in response to changes in various recording conditions. Hereinafter, resistance against an alteration in the color density and unevenness in the color density caused by the shift of the recording positions of the planes, resulting from any recording condition change, is referred to as “robustness”.
Japanese Patent Laid-Open No. 2000-103088 discloses an image data processing method for increasing the robustness. Japanese Patent Laid-Open No. 2000-103088 focuses on a fact that a change in image color density caused by a change in various recording conditions can result from a complete mutual complementary relationship of pieces of binary image data for different recording scans. As understood from this document, it is considered that multipass printing having superior “robustness” can be realized if pieces of image data for different recording scans are generated so that the degree of the complementary relationship therebetween is reduced. Accordingly, in Japanese Patent Laid-Open No. 2000-103088, multivalued image data is divided before binarization and the divided pieces of multivalued image data are then separately binarized. In this manner, a significant color density change may be prevented even if image data of different planes corresponding to different recording scans are recorded at shifted positions.
FIGS. 3A-3I are diagrams for describing a data division method disclosed in Japanese Patent Laid-Open No. 2000-103088. First, multivalued image data (see FIG. 3A) to be recorded on a unit area is divided into multivalued data (see FIGS. 3B and 3D) to be recorded in the first pass, and multivalued data (see FIGS. 3C and 3E) to be recorded in the second pass. Each piece of the multivalued data is separately binarized (see FIGS. 3F and 3G), whereby binary data (see FIG. 3H) to be recorded in the first pass, and binary data (see FIG. 3I) to be recorded in the second pass, are generated. Lastly, ink is ejected from a recording head in accordance with these pieces of binary data. As is clear from FIGS. 3H and 3I, the binary data for the first pass and the binary data for the second pass that are generated in the above-described manner may not have a complete complementary relationship. Accordingly, parts where dots of the first pass and dots of the second pass overlap (pixels having “1” in two planes) and parts where dots of the first pass and dots of the second pass do not overlap (pixels having “1” in only one of the planes) coexist.
FIG. 4 is a diagram showing an arrangement of dots recorded on a recording medium in accordance with the method disclosed in Japanese Patent Laid-Open No. 2000-103088. Referring to the drawing, black circles 21 represent dots recorded in the first pass, whereas white circles 22 represent dots recorded in the second pass. Shaded circles 23 represent overlapping dots recorded in both of the first and second passes. In this example, since the complementary relationship between the first pass and the second pass is incomplete, parts where two dots overlap and parts where no dot is recorded (blank areas) exist, which is different from a case of having a complete complementary relationship as shown in FIG. 2.
As in the case of FIG. 2, a case is considered where positions of dots recorded in the first pass, and positions of dots recorded in the second pass, are shifted by an amount equivalent to one pixel in a main scanning direction or a sub scanning direction. In this case, the dots of the first and second passes, which are not supposed to overlap unless the positional shift is caused, do overlap. On the other hand, the dots 23, which are supposed to overlap unless the positional shift is caused, do not overlap. Accordingly, the coverage ratio of the dots to the blank area does not change as much, and a change in color density of the image is less in an area having a predetermined size. Therefore, even if an alteration in a distance (paper distance) between a recording medium and an ejection orifice surface, and an alteration in a conveyed distance of the recording medium are caused, the method disclosed in Japanese Patent Laid-Open No. 2000-103088 may suppress a change in image color density caused by these alterations.
Furthermore, Japanese Patent Laid-Open No. 2006-231736 discloses a technique for distributing, like Japanese Patent Laid-Open No. 2000-103088, multivalued image data to a plurality of recording scans or a plurality of recording element lines, while changing a data distribution ratio on the basis of the positions of pixels. Japanese Patent Laid-Open No. 2006-231736 describes the advantage of suppressing banding and color unevenness that may be caused in multipass printing by changing the distribution ratio linearly, periodically, like a sine wave, or like a combined wave of a high-frequency wave and a low-frequency wave, with respect to a position in a main scanning direction.
Although the methods (hereinafter, referred to as a “multivalued data division method”) disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2006-231736 provide robustness that may be superior to that provided by the mask-employing division method, the methods may also have disadvantages as compared to the mask-employing division method. For example, the image color density may be more likely to become low in the multivalued data division method than in the mask-employing division method because of the lower dot coverage ratio. In addition, since a blank area may be created, as shown in FIG. 4, a contrast and sharpness of images may be more likely to decrease.
Thus, when images are recorded while emphasizing the color density, the contrast, and the sharpness rather than the robustness, the mask-employing division method may be employed in some cases rather than the multivalued data division method. Since a division method may differ depending on a content(type) of image data in this manner, in certain instances it may not be as effective to employ the multivalued data division method, regardless of the content (type) of the image data.